Certainly the Scripture says that THERE IS ONLY ONE GOD...but it doesn't say you have to guess who he is, because it specifically states WHO THAT GOD IS:
1 Cor. 8:4 (...) we know that an idol is nothing in the world, and that there is no God but one. 5 For even though there are those who are called “gods,” whether in heaven or on earth, just as there are many “gods” and many “lords,” 6 there is actually to us one God the Father, out of whom all things are, and we for him; and there is one Lord, Jesus Christ, through whom all things are, and we through him.
Whoever has another god or more, or a group of gods that he imaginatively numbers as one, is simply part of that world that is spoken of there, which has many other different gods. Jesus himself said that the Father is the only true God (John 17:3).
Trinitarians are rebellious and disobedient people, who don't even pay attention to Jesus, even if they think they are Christians.
Deut. 6:4 “Listen, O Israel: Jehovah our God is one Jehovah. 5 You must love Jehovah your God with all your heart and all your soul and all your strength."